1. Field of the Invention
The present invention relates in general to digital communication systems, and in particular, to mobile radio digital communication systems. Still more particularly, the invention relates to a method of reception in a receiver of a mobile radio digital communication system.
2. Description of the Related Art
The most widespread standard in cellular wireless communications is currently the Global System for Mobile Communications (GSM). GSM employs a combination of Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) for the purpose of sharing the spectrum resource. GSM networks typically operate in the 900 MHz and 1900 MHz frequency ranges. For example, GSM-900 commonly uses radio spectrum in the 890-915 MHz bands for the uplink (Mobile Station to Base Transceiver Station) and in the 935-960 MHz bands for the downlink (base station to mobile station), providing 124 RF channels spaced at 200 kHz, and GSM-1900 uses the 1850-1910 MHz bands for the uplink and 1930-1990 MHz bands for the downlink. The spectrum for both uplink and downlink is divided into 200 kHz-wide carrier frequencies using FDMA, and each base station is assigned one or more carrier frequencies. Each carrier frequency is divided into eight time slots using TDMA such that eight consecutive time slots form one TDMA frame with a duration of 4.615 ms. A physical channel occupies one time slot within a TDMA frame. Each time slot within a frame is also referred to as a “burst.” TDMA frames of a particular carrier frequency are numbered and formed in groups of 26 or 51 TDMA frames called multi-frames.
GSM systems typically employ one or more modulation schemes to communicate information such as voice, data, and/or control information. These modulation schemes may include GMSK (Gaussian Minimum Shift Keying), M-ary QAM (Quadrature Amplitude Modulation) or M-ary PSK (Phase Shift Keying), where M=2n, with n being the number of bits encoded within a symbol period for a specified modulation scheme. The most common modulation scheme, GMSK, is a constant envelope binary modulation scheme allowing raw transmission at a maximum rate of 270.83 kilobits per second (Kbps).
Wireless communication systems are placing an ever-increasing demand on capacity to transfer both voice and data services. While GSM is efficient for standard voice services, high-fidelity audio and data services demand higher data throughput rates. The General Packet Radio Service (GPRS), EDGE (Enhanced Data rates for GSM Evolution) and UMTS (Universal Mobile Telecommunications System) standards have been adopted to increase capacity in GSM systems.
General Packet Radio Service (GPRS) is a non-voice service that allows information to be sent and received across a mobile telephone network. It supplements Circuit Switched Data (CSD) and Short Message Service (SMS). GPRS employs the same modulation schemes as GSM, but higher data throughput rates are achievable with GPRS since it allows for an entire frame (all eight time slots) to be used by a single mobile station at the same time.
EDGE (Enhanced Data rates for GSM Evolution) and the associated packet service EGPRS (Enhanced General Packet Radio Service) have been defined as a transitional standard between the GSM/GPRS (Global System for Mobile Communications/General Packet Radio Service) and UMTS (Universal Mobile Telecommunications System) mobile radio standards. Both GMSK modulation and 8-PSK modulation are used in the EDGE standard, and the modulation type can be changed from burst to burst. GMSK is a non-linear, Gaussian-pulse-shaped frequency modulation, and 8-PSK modulation in EDGE is a linear, 8-level phase modulation with 3π/8 rotation. However, the specific GMSK modulation used in GSM can be approximated with a linear modulation (i.e., 2-level phase modulation with a π/2 rotation). The symbol pulse of the approximated GSMK and the symbol pulse of 8-PSK are identical.
In GSM/EDGE, frequency bursts (FB) comprising a single tone, which corresponds to all “0” payload and training sequence, are sent regularly by the Base Station (BS) to allow Mobile Stations (MS) to synchronize their Local Oscillator (LO) to the Base Station LO, using frequency offset estimation and correction. The all zero payload of the FB is a constant frequency signal, or a single tone burst. When the MS is power mode, it hunts continuously for a FB from a list of carriers. When the FB is detected, the MS will estimate the frequency offset relative to its nominal frequency, which is 67.7 KHz from the carrier. This estimated frequency offset will be used to correct the MS LO. In power up mode, the frequency offset can be as much as +/−19 KHz. In a standby mode, the MS will periodically wakeup to monitor the frequency burst to maintain its synchronization. In the standby mode, the frequency offset is within ±2 KHz.
While detecting the presence of a FB and estimating the frequency of the FB single tone, the MS also estimates the starting or ending time of the FB to provide a coarse timing of the FB. This coarse timing is used to locate a synchronization burst, called a SCH burst, that follows the detected FB. The synchronization burst is used to derive a fine tuned timing to schedule the following normal burst processing. The SCH also provides a base station ID. As can be seen, frequency channel burst detection is an important function of GSM/EDGE phones.
The FB sent from the base station is a single tone at ¼ symbol rate or 67.7 KHz relative to the carrier frequency. The base station carrier frequency is specified to be better than 0.05 ppm or less than ±45 Hz uncertainty for 900 MHz carriers and less than ±90 Hz uncertainty for 1900 MHz carriers. For a 10 ppm MS local oscillator, a FB is observed at MS as a 67.7 KHz tone with a ±19 KHz uncertainty for 1900 MHz carriers.
If the received signal is sampled at the symbol rate of 270.83 KHz, the nominal frequency of the FB will be at ¼ of the sampling rate. Thus, in the normalized frequency domain, the nominal frequency of the FB tone will be exactly at ½π. The frequency uncertainty is ±19 KHz in power-up mode and ±2 KHz in standby mode, which are equivalent to the uncertainty of ±25.26° and ±2.66° respectively in the normalized frequency domain. Accordingly, the frequency search window for FB detection is centered at ½π with a width of ±25.26° and ±2.69° in the power-up mode and standby mode respectively, and the estimation error is required to be less than 100 Hz or 0.0665°.
In GSM/EDGE, FBs are allocated through the Frequency Correction CHannel (FCCH), as shown in FIG. 1. A FCCH burst in GSM/EDGE has a duration of 576.92 us, which is equivalent to 156.25 symbol periods with a 270.83 KHz symbol rate. As explained above, the FB is a single tone signal at 67.7 KHz relative to the carrier center, and the carrier center of the received signal could have a variation between ±19 KHz due to the MS's LO uncertainty in power-up mode and a variation within ±2 KHz while in standby mode.
FIG. 1 shows the Frequency Correction CHannel (FCCH) structure within each Multi-Frame transmitted over the channel. The Multi-Frame 90 has 51 TDMA frames, including five TDMA frames carrying frequency bursts (FBs). As exemplified in FIG. 1 by the expanded frame 20, each Frame is shown with its eight time slots 0-7. The first time slot of each of the 0, 10, 20, 30 and 40th TDMA Frames contains the FB. As further exemplified in FIG. 1 by expanded time slot 0 of frame 20, each FB contains 142 “0” bits in the place of payload and training sequence in normal bursts, and three tail bits at both the front and end of the FB, which are also “0” bits. FIG. 1 also shows frame 41 of the multi-frame expanded to show eight time slots 0-7, including the synchronization channel (SCH) at time slot 0 of frame 41.
Detection of the FB is complicated by the fact that the burst has a wide frequency uncertainty window (±19 KHz) relative to 200 KHz bandwidth of the channel. Consequently, detecting the presence of the FB and its arrival time has previously required multifaceted electronic circuits that significantly impact the performance of the subsequent normal burst recovery and power consumption. Conventional FB detectors use a pole-adaptation approach operating on the real-valued signal to detect the presence of the single tone burst. The tone signal power is estimated and compared with the total signal power received. Estimating the tone power relies on the tone frequency estimation derived from phase differences of neighboring samples of the received signal. Pole adaptation attempts to drive an IIR filter to be centered at the tone frequency. However, this pole adaptation is based on localized signal statistics, thus is subject to noise and interference disturbance. Considering the fact that the received signal always contains the receiver thermal noise and interference, very complicated circuitry is involved in the pole adaptation. Even with the additional circuitry, the solution is still limited due to the fundamental limitations with the approach. It is observed that the prior art solutions result in high power consumption and low performance, especially in harsh fading environment. What is needed is a low cost, low power consumption and high performance FB detection solution that will improve the consumer experience.
In the accompanying drawings, elements might not be to scale and may be shown in generalized or schematic form or may be identified solely by name or another commercial designation.